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Types of Diode and their Description(Tunnel Diode)

                    Tunnel Diode


The tunnel diode is a form of very fast semiconductor diode that can operate well into the microwave radio frequency region.
It differs from other forms of semiconductor diode in that it uses a quantum mechanical effect called tunnelling. This provides the tunnel diode with a negative resistance region in its IV characteristic curve that enables it to be used as an oscillator and as an amplifier.
Although they are not as widely used as some devices today, these devices do have their place within RF technology. They were used in television receiver front end oscillators and oscilloscope trigger circuits, etc. They have been shown to have a very long life and can offer a very high level of performance when used as an RF pre-amplifier. However today, their applications are often limited because more traditional three terminal devices can offer a better level of performance in many areas.

Tunnel diode development

The tunnel diode was discovered by a Ph.D. research student named Esaki in 1958 while he was investigating the properties of heavily doped germanium junctions for use in high speed bipolar transistors.
In the course of his research Esaki produced some heavily doped junctions for high speed bipolar transistors and as a result he found that they produced an oscillation at microwave frequencies as a result of the tunnelling effect.
Then in 1973, Esaki received the Nobel prize for Physics for his work on the tunnel diode.
After the work by Esaki, other researchers demonstrated that other materials also showed the tunnelling effect. Holonyak and Lesk demonstrated a Gallium Arsenide device in 1960, and others demonstrated Indium tin, and then in 1962 the effect was demonstrated in materials including Indium Arsenide, Indium Phosphide and also Silicon.

Tunnel diode circuit symbol

Despite the operation of the tunnel diode. its circuit symbol is based on that for the standard diode, but has 'tails' added to the bar element of the symbol to differentiate it from other forms of PN junction diode.
The circuit symbol for the tunnel diode showing the slightly different arrangement on the bar to differentiate from other forms of diode
Tunnel diode circuit symbol

Advantages and disadvantages

The tunnel diode is not as widely used these days as it was oat one time. With the improvement in performance of other forms of semiconductor technology, they have often become the preferred option. Nevertheless it is still worth looking at a tunnel diode, considering its advantages and disadvantages to discover whether it is a viable option.
Advantages
  • Very high speed:   The high speed of operation means that the tunnel diode can be used for microwave RF applications.
  • Longevity:   Studies have been undertaken of the tunnel diode and its performance has been shown to remain stable over long periods of time, where other semiconductor devices may have degraded.
Disadvantages
  • Reproducibility:   It has not been possible to make the tunnel diode with as reproducible performance to the levels often needed.
  • Low peak to valley current ratio:   The negative resistance region and the peak to valley current is not as high as is often be required to produce the levels of performance that can be attained with other devices.
One of the main reasons for the early success of the tunnel diode was its high speed of operation and the high frequencies it could handle. This resulted from the fact that while many other devices are slowed down by the presence of minority carriers, the tunnel diode only uses majority carriers, i.e. holes in an n-type material and electrons in a p-type material. The minority carriers slow down the operation of a device and as a result their speed is slower. Also the tunnelling effect is inherently very fast.
The tunnel diode is rarely used these days and this results from its disadvantages. Firstly they only have a low tunnelling current and this means that they are low power devices. While this may be acceptable for low noise amplifiers, it is a significant drawback when they are sued in oscillators as further amplification is needed and this can only be undertaken by devices that have a higher power capability, i.e. not tunnel diodes. The third disadvantage is that they are problems with the reproducibility of the devices resulting in low yields and therefore higher production costs.

Applications

Although the tunnel diode appeared promising some years ago, it was soon replaced by other semiconductor devices like IMPATT diodes for oscillator applications and FETs when used as an amplifier. Nevertheless the tunnel diode is a useful device for certain applications.
Applications for the tunnel diode included uses as an oscillator, although it was also used as an amplifier and a mixer.
One of the major advantages of the tunnel diode which is currently beginning to be experienced is its longevity. Once manufactured its performance remains stable over long periods of time despite its use. Other devices might degrade slightly over time.


While the tunnel diode is a semiconductor device using the same materials as other forms of diode and active devices, the very high levels of dopant used, cause the devices to operate in a very different manner.
The device theory shows that it does not act as a diode, but instead exhibits a negative resistance region in the forward direction.
The IV characteristic curve, combined with the very high speed of the diode means that the it can be used in a variety of microwave RF applications as an active device.

Tunnel diode theory basics

The characteristic curve for a tunnel diode shows an area of negative resistance. When forward biased the current in the diode rises at first, but later it can be seen to fall with increasing voltage, before finally rising again.
It is also interesting to note that current also flows in the reverse direction - the reverse breakdown voltage is actually zero and the diode conducts in the reverse direction. The characteristics near the original are virtually symmetrical.
The IV characteristic of the tunnel diode showing the important voltage turning points and the negative resistance region
Tunnel diode IV characteristic
The reason for this is that there are a number of different components to forming the overall curve.
  • Normal diode current:   This is the 'normal' current that would flow through a PN junction diode.
  • Tunnelling current:   This is the current that arises as a result of the tunnelling effect.
  • Excess current:   This is a third element of current that contributes to the overall current within the diode. It results from what may be termed excess current that results from tunnelling though bulk states in the energy gap, and means that the valley current does not fall to zero.
The different current contributions or components that make up the overall current passed by the tunnel diode
Tunnel diode current components
These three main components sum together to provide the overall level of current passed by the tunnel diode.

Tunnelling mechanism & theory

Tunnelling is an effect that is caused by quantum mechanical effects when electrons pass through a potential barrier. It can be visualised in very basic terms by them "tunnelling" through the energy barrier.
The tunnelling only occurs under certain conditions. It occurs within tunnel diodes because of the very high doping levels employed.
At reverse bias, the electrons tunnel from the valence band in the p-type material to the conduction band in the n-type material, and the level of the current increase monotonically.
For the forward bias situation there are a number of different areas. For voltages up to Vpe, electrons from the conduction band find increasing availability of empty states in the valence band and the level of current increases up to a point where the current equals Ipe.
Once this point is reach, it is found that number of empty states available for electrons with the level of energy they are given by the increased voltage level starts to fall. This means that the current level falls in line with this. The overall current level falls away relatively swiftly, dropping to near zero.
As the current from the tunnelling effect falls, so the diffusion current, which is the same action as occurs in a normal PN junction diode starts to increase and steadily becomes the dominant mechanism.

Tunnel diode characteristics

The diagram towards the top of the page shows the tunnel diode IV characteristic. This has a form of 'N' shaped curve. With an area of negative resistance between the peak voltage, Vpe and the valley voltage Vv.
The values for these voltages depend upon the diode material and also upon its individual characteristics.

TUNNEL DIODE PROPERTIES FOR DIFFERENT MATERIALS
PARAMETERGERMANIUMGALLIUM ARSENIDESILICON
Vpe (mV)40 - 7090 -12080 - 100
Vv (mV)250 - 350450 -600400 - 500
Ipe/Iv10 -1510 - 203 - 5
One of the useful figures of merit for a tunnel diode characteristic is the peak to valley current ratio, Ipe / Iv. From the values in the table it can be seen that silicon has a very low value and as a result, this means that it is not normally one of the best options for a tunnel diode.

The basic structure of the tunnel diode is similar to that of an standard PN junction.
However there are a few key differences that enable the tunnel diode to operate differently to the ordinary PN junction.
Even so, the basic processes and fabrication techniques used with main-line semiconductor technology can be used for the tunnel diode.

Tunnel diode structure basics

The tunnel diode is similar to a standard p-n junction in many respects except that the doping levels are very high. Densities of the order of 5x10^19 cm^-3 are common.
A further difference is that the depletion region, the area between the p-type and n-type areas, where there are no carriers is very narrow. Typically it is in the region of between five to ten nano-metres, which equates to a width of only a few atoms.
As the depletion region is so narrow this means that if it is to be used for high frequency operation the diode itself must be made very small to reduce the high level of capacitance resulting from the very narrow depletion region.
In terms of the material used for these diodes, the favoured semiconductor is germanium. Although other materials can be used and have been used, germanium has the advantage that it has a small energy gap that allows for more efficient tunnelling.

Tunnel diode fabrication structures

Tunnel diode structures generally fall into one of three basic structures:
  • Ball alloy:   This type of tunnel diode format is fabricated as a mesa structure. To achieve this form of structure, the fabrication technique involves bringing an alloy containing the required dopants into contact with a heavily doped substrate. The temperature used is around 500°C at which point the dopants quickly melt and diffuse into the substrate. The overall structure geometry is then defined by etching the diode to the required proportions.

    The tunnel diode device physical structure for the ball alloy method of fabrication
    Tunnel diode ball alloy structure

  • Pulsed bond:   This is a relatively straightforward structure to create, although careful process control is required during the fabrication process. The diode is created by using a wire coated with an alloy containing the required dopants. This is pressed hard onto the heavily doped substrate, and then a voltage pulse is applied. The effect of this is that the junction forms by a process of local alloying. 

    The tunnel diode device physical structure for the pulsed bond method of fabrication
    Tunnel diode pulsed bond structure


    Despite this, there are drawbacks to this process because it can only produce a small junction, and the exact properties, including the area of the junction cannot be controlled tightly.
  • Planar structure:   Planar technology can be used to create the diode. Using this approach for the fabrication process, the heavily doped n+ substrate is masked off by an insulating layer to leave a small area exposed. This exposed area is then open to become the active area of the diode.

    The tunnel diode device physical structure for the planar method of fabrication
    Tunnel diode planar structure


    The doping for the area can be introduced by one of a number of means. It can be introduced by diffusion, alloying or epitaxial growth. Alternatively it is possible to grow an epitaxial layer over the whole surface and then etch away those areas that are not required to leave a mesa structure.
All three structures enable high performance diodes to be obtained.
Although these are three popular structures for tunnel diodes, new developments are occurring using different materials and also involving new structures that offer a greater variety of characteristics, or they may be tailored to the needs of a particular material that may be used.

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